the different output devices, such as PIA chip, POKEY chip and the
CTIA chip.
Only four of the possible output selections are used. ( See Table 4-
3. ) The I/O Decoder is enabled by inputs on pins 4, 5, and 6; pin 4
input coming from the Memory Map Decoder (S6); pin 5 input coming the
Address Bus and pin 6 input coming from the Chip Select (CS) line.
The Binary Decode is provided by the Address Bus (A08, A09 and A10).
Table 4-3. I/O Decoder Select Lines
Enable/Pin Binary Decode
A
1
2
6
A
1
1
5
S
6
4
A
1
0
3
A
9
2
A
8
1
Active
Pin
Device
Selected
H
L
L
L
L
L
Y0 - 15
CTIA
H
L
L
L
L
H
Y1 - 14
Not Used
H
L
L
L
H
L
Y2 - 13
POKEY
H
L
L
L
H
H
Y3 - 12
PIA
H
L
L
H
L
L
Y4 - 11
Not Used
H
L
L
H
L
H
Y5 - 10
External
Select
Y6 - 9
Not Used
Y7 - 8
Not Used
ATARI Personal Computers 4-13
4.3.3 Motherboard Console System (800)
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